Tunnelling Techniques: Recent experience in excavations in complex geological conditions

Recent experience in excavations in complex geological conditions

With the growing number and increasing complexity of railway projects in India, the demand for tunnel development has gained momentum in the past few years. Innovative technologies are being induc­ted as railway projects involving longer tunnel lengths and varying geological challenges are being developed across the country.

The Indian Railways’ network is the fourth largest rail network in the world and runs throu­gh a diverse topography. The Himalayan region poses various challenges in the construction of tunnels. Not only is access difficult in this re­mo­te region, but its topography is un­suitable for construction activity. Despite this, several projects are ongoing in the Hima­layas and low­er Hi­ma­layas, including the Udhampur-Sri­na­gar-Bara­mulla rail link, Jiribam-Tupul-Imphal link, Tetelia-Byrnihat-Shillong link, Bhai­ra­bhi-Sairang link, Sivok-Rangpo link, Rishike­sh-Karan­prayag link and the Bhanupali-Bilas­pur-Beri new railway line project.

Excavation using tunnel boring machines (TBMs) has been quite successful in constructing metro, water supply and sewerage tunnels. However, the high cost of TBMs and the difficulty in mobilising the machines to job sites, especially in the Himalayan region, remain big deterrents. Flooding and mucking have resulted in TBMs getting buried, leading to time and cost escalations. Hence, in the excavation of railway tu­nnels in difficult geological conditio­ns, the New Austrian Tunnelling Method (NATM) has emerged as a suitable alternative.

Unlike the conventional methods, where the tunnel is immediately supported without all­o­wing it to deform naturally, NATM allows the deformation of the rock mass to take place before stabilising the tunnel, thereby reducing the amount of additional support material re­quired. In the world of tunnelling, this meth­od is seen as a set of principles rather than a technical method.

Recent trends in railway tunnel excavation

Explosives are the most important raw material used in rail tunnel construction. Broadly, the four variants of explosives used are low explosives, high explosives, packaged explosives and bulk explosives. The second most important raw ma­terial used in rail tunnel construction is steel. This is primarily used for steel rib supports, lattice girders, mesh reinforcement, rock bolting and anchoring, and self-drilling anchors.

Over the years, tunnel contractors have also started using a metakolin mineral admixture for high strength concrete and grouting. Self-compacting concrete has also emerged as a cost-effective option for casting heavily reinforced elements and complex geometrical sha­p­es. Besides, retarder chemical is being used to increase the setting time of cement slurries. Geotextile membranes are also being used to make tunnels waterproof.

The Udhampur-Srinagar-Baramulla rail link pro­ject is one of the largest projects in the region. It has a total route length of 272 km consisting of 930 bridges and 39 tunnels with a total length of 13,008 metres and 116 km res­pectively. The project involved various key decisions such as selection of alignment, choi­ce of the cross section, layout of long tunnels and construction material to be used. Such decisions are vital for successful implementation and smooth operation of the project. An elliptical shaped section has been ad­opted for the cross section of tunnels in the Hi­malayan region as the horizontal load is grea­ter than the vertical load.

The Bhanupali-Bilaspur-Beri new railway line project is one such project that involves the construction of various tunnels under challenging geological conditions. The project is located at the foothills of the Himalayas in Punjab and Himachal Pradesh and will have a total length of 63.10 km, of which 26 km will be tunnel length. Around 20 tunnels have been planned as part of the project, with the longest tunnel having a length of 4 km. Construction work has commenced on seven tunnels.

The project is being developed in challenging geological conditions, as tunnels for the railway line are located in the lower Siwalik formation and are being constructed through ground that has a gradient range of 1:12. The tectonic location of the project falls between the main boundary thrust, named the Jammu thrust, and the main frontal thrust.  In order to ensure successful construction of tunnels in this project, NATM was deployed as it allows a greater degree of flexibility at the excavation stage.

New types of material are also being used to improve the durability and strength of tunnels under the project. Steel fibre-reinforced concrete is being used in the tunnel construction as it provides ductility in tension and compression as well as high resistance against the spalling of concrete. Micro silica or micro fine cement is being used in pre-grouting to control water seepage. Further, synthetic fibres with micro silica are being deployed in shotcreting as it improves strength and reduces rebound. Artificial ground freezing is being used for soft ground tunnelling to create a watertight barrier as a support system.

Technological advancements

A notable development in the construction of tunnels in difficult terrain has been the use of smart machines. In the construction of tunnels in the Bhanupali-Bilaspur-Beri new railway line project, smart machines were deployed for various construction activities. The use of these machines accelerated the excavation work, re­du­ced the turnaround time, avoided overcuts and undercuts, and improved blasting efficiency. Hence, utilisation of smart machines is the way forward for construction in difficult topography.

With regard to instrumentation in the Bha­nupali-Bilaspur-Beri new railway line project, 3D monitoring targets have been installed clo­se to the cavity area of the tunnel in order to observe further deformation and settlement inside the tunnel. These 3D monitoring targets also ensure the safety of construction workers. Based on the 3D monitoring data and its stable trend of deformation for several days, further activity is resumed on the project.

Key issues, challenges and future outlook

Geology impacts almost every aspect of a rail tunnelling project, from conception to commissioning. Most rail tunnelling projects in India face significant issues arising from geological surprises and inadequate investigations. Iss­ues such as thrust and shear tectonic zones, hot water springs or thermal zones, saturated strata consisting of lake and terrace deposits, and trapped methane gas pose significant cha­llenges to tunnel contractors. Therefore, detail­ed geological investigations are needed to determine the construction methodology and equipment requirement for projects. However, in their haste to issue tenders, developers do not undertake such geological investigations properly. Instead, they rely on surface geological mapping to interpret the region’s ge­o­logy. In this case, unexpected geological com­plexi­ties can impose huge financial risks and time delays on railway projects.

Another key issue in tunnel excavations for railway projects is the weak integrity of contract documents and practices. Ambiguous contracts are often the reason for disputes in railway tunnelling projects. For instance, the roles, responsibilities and risk sharing among contract parties are often not clearly defined in the documents. Further, there is a lack of clarity on the methodology to be used for working out the rates for extra items and for deviations in design or construction requirements.

Railway projects involving the excavation of tunnels in India are riddled with sector-specific challenges of heightened safety risks and geological uncertainties. Several projects have fa­ced the brunt of these risks, resulting in project delays, cost escalations, loss of lives, and en­hanced financial burden. At the same time, the poor quality of enforcement contracts in the country, coupled with weak institutional mechanisms to deal with contract disputes, adds to contractors’ woes. Thus, appropriate measures need to be taken by project developers, tunnel contractors and appropriate government de­part­ments to mitigate such risks. Addressing these risks is expected to spur growth in the tunnelling sector.

Based on a presentation by Rajeev Soni, Chief Project Manager, Rail Vikas Nigam Limited, at a recent Indian Infrastructure conference